Funny you ask. I run an Alesis quadaverb and plug it into the effects loop on the Marshall. This enables me to crank the amp but use the volume controls on the effects unit to control the volume. I talked to Marshall about this and there is nothing wrong with doing this. The effects loop is a ciruit between the preamp section and power sections of the amp so according to Marshall there is no detriment to amp what so ever and it is designed to be used like this. However any time you crank the master volume you are beating up your output valves.
Using this seup I can crank the pre section and get the gain, use the volume on effects processor to keep the volume in check and send a high gain signal with low volume to the power section resulting in decent tone at lower volumes. A lower end effects processor such as this will kill some of your tone but I find EQing compensates ok. Other then that I use an old DOD overdrive pre-amp pedal with the level knob at about half and the distortion knob just about set at full cut. The rest is the effects processor to add echo and some EQ. I use a Gibson Les Paul Custom with a different bridge pickup. Sorry, it was like 20 years ago I swapped out the pickup so I don't remember what kind it is. Some kind of high gain Dimarzio if memory serves me.
Very simple setup. The way I like it.
This is what I do. Your results may vary.
Another thing that makes a huge difference is the valves your using in your amp. I just had my Marshall re-valved because they had never been changed in like 10 years. Marshalls come stock with Svetlanas these days and back when my amp was made in the late 80's I am told they came with either Svetlanas or Teslas. I had my amp re-valved with Groove Tubes because that is what the shop I went to uses and it changed everything, for the worse. I ended up putting one of my old pre valves in the first position (this is the one that controls tone on Marshalls) to get some of my sound back. I am next going to order a set of Svetlana Pre valves to see if I can get my old sound back.
I wrote a whole a editorial and provided several links about this in the thread New AMP? if your interested in perusing it for more in depth information. I have provided the link to it.
http://forum.bandmix.com/viewtopic.php?t=3845&start=15
Scroll down to the last entry which is me and the information is there.